In general, the optical membranes are applied in the backlight modules to enhance the brightness of the backlight modules, wherein the membrane needs to be positioned by certain designed mechanism due to its small thickness. The existing optical membranes are provided with a plurality positioning holes that have figures matching with those of the positioning poles. However, an expansion and contraction effect might occur to the optical membranes in high temperature and humidity conditions, and an interference phenomenon of the optical membranes would appear between the positioning posts and the positioning holes in where they are positioned. As a result, the optical membranes could be deformed and curved, which greatly affect the installation and using of the optical membranes, thereto reduce imaging quality of the backlight modules by appearing shadow and other inferior phenomenon.